(1) Field of the Invention
This disclosure relates generally to the field of monitoring of battery characteristics, and more particularly to battery monitoring based on measurement of a differential current (or inter-battery current) between batteries that are connected in parallel.
(2) Description of the Related Art
Batteries are made of one or more cells, for example lithium-ion cells, and are used to power many different types of devices. Catastrophic failures of batteries have resulted in large-scale recalls costing hundreds of millions of dollars and in significant damage to company reputation and brand image. As the energy content of batteries continues to rise, the potential for serious failures becomes a major concern. Catastrophic battery failure may include a thermal runaway event in which an internal short circuit inside a cell initiates a self-accelerating decomposition reaction inside the cell. Thermal runaway events may include smoke, flames, or even an explosion if intervention is not performed in a timely manner.
Several different approaches are available to detect short circuits in battery cells by monitoring cell or cell block voltages. In these approaches, cell or cell block voltage changes are monitored when no charging or discharging is taking place in the cell in order to detect a drop in voltage across the cell that may be associated with an internal short circuit. Monitoring of cell voltage over time while a cell is at rest (i.e., when there is no charge or discharge current flowing in the cell) is used in the lithium-ion industry by cell manufacturers as a quality control test. Following assembly of the cell and initial charging, cells are left at rest for a period of time and cell voltage is monitored. A decline in cell voltage beyond a certain value can indicate the presence of an internal short in the cell, thus allowing the cell to be rejected as faulty. Such voltage tests do not identify cells that will develop internal short circuits later in their life cycle, which may lead to catastrophic failures of cells that develop internal short circuits during operation. Further, monitoring of cell voltage when a cell is at rest has a relatively low sensitivity with respect to detection of internal shorts because an internal short may require an extended period of time to have a sufficiently large impact on cell voltage to allow unambiguous confirmation of whether an internal short is present in the cell. If a short is allowed to develop too far, a point-of-no-return may be reached where it is no longer possible to avoid thermal runaway and catastrophic cell failure.